In recent years, many vehicles are provided with a navigation apparatus that provides travel guidance for the vehicle to allow a driver to easily reach a desired destination location. The navigation apparatus is a device that can detect the current position of the vehicle using a GPS receiver or the like and acquire map data corresponding to the current position from a storage medium such as a DVD-ROM or an HDD to display the map data on a liquid crystal monitor. The navigation apparatus reads map data including the current position of the vehicle from the storage medium or the like and depicts an image of a map around the current position of the vehicle on the basis of the map data to display the map image on a display device, displays a vehicle position mark as superposed on the map image, scrolls the map image in accordance with movement of the vehicle, and moves the vehicle position mark with the map image stationary on the screen so that it can be seen at a glance what location the vehicle is currently traveling at. The navigation apparatus also has a route search function for exploring for an optimum route from a departure location (e.g. the current position of the vehicle) to a set destination location when a desired destination location is set, and further has a travel guide function for providing travel guidance along the route found in the search. In recent years, in addition, some cellular phones, PDAs (Personal Digital Assistants), personal computers, and so forth also have functions that are similar to those of the navigation apparatus described above.
New roads (newly constructed roads) are constructed all over Japan every year. Along with that, existing roads are dismantled or changed in shape. In this event, information on the newly constructed roads or the like newly built after the map data of the navigation apparatus or the like are generated is not registered in the map data. That is, in the navigation apparatus or the like which has map data not including the newly constructed roads or the like, the newly constructed roads are not displayed on the liquid crystal monitor, or not used in exploring for a route or providing travel guidance. Therefore, the navigation apparatus or the like may find a roundabout guide route that does not pass through the newly constructed roads or the like. In order to avoid such an inconvenience, it is necessary to update the map data stored in the navigation apparatus or the like at certain time intervals. The map data are updated by purchasing a storage medium for new map data to replace the old storage medium, or rewriting the map data of the navigation apparatus or the like on the basis of map data for update (hereinafter referred to as “map updating data”) distributed from a server such as a map information distribution center.
In a method for updating map data stored in a navigation apparatus or the like using map updating data distributed from a server such as a map information distribution center, in particular, in order to reduce the amount of data for communication, difference update in which only a part of information that needs to be updated such as information related to newly constructed roads is updated is performed (see Japanese Patent Application Publication No. 2007-328739 (JP 2007-328739 A) and Japanese Patent Application Publication No. 2010-9390 (JP 2010-9390 A), for example).